The RGS wavelength scale is determined by the relative alignment of the various instruments and their structural stability. Studies of spectra of emission line sources have shown that line positions are systematically shifted with respect to laboratory wavelengths. These shifts have been found to be correlated with the angular distance between the spacecraft pointing direction and the Sun (“solar angle”). The correction of this dependence aligns the wavelength scales of both spectrographs and orders, and improves substantially their accuracy. Fig. 83 shows the residuals of the RGS measured versus nominal positions for certain emission lines and a set of observations of coronal sources. For an on-axis source, the wavelength scale accuracy is 5 mÅ, after application of the heliocentric velocity correction and removal of the solar angle dependence. It has been verified that the wavelength scale is stable over a long timescale.
European Space Agency - XMM-Newton Science Operations Centre